1. Field of the Invention
This invention relates to a process for producing a powdery, melamine-coated ammonium polyphosphate. More particularly, it relates to a process for producing a powdery, melamine-coated ammonium polyphosphate used as a component of flame-retardants to be added to or impregnated in thermoplastic resins, paints, etc. or paper, etc.
2. Description of the Related Art
Ammonium polyphosphate is very easily hydrolyzed due to its chemical structure; hence when it is used as a component of flame-retardants by adding it to or impregnating it in thermoplastic resins, paints, etc. or paper, etc., there are raised drawbacks that various characteristics of the resulting product, such as electrical insulation properties, mechanical physical properties and flame-retardancy are notably lower due to the hygroscopicity, water-solubility and hydrolytic properties.
In order to overcome these drawbacks, a number of proposals have so far been made as regards the improvements in water-solubility and hydrolytic properties of ammonium polyphosphate. For example, Japanese patent publication No. Sho 53-15478 discloses a process wherein 5 to 50% by weight of melamine or melamine phosphate as a melamine component, ammonium phosphate and urea or crystalline urea phosphate are mixed, followed by heat-condensation, to produce a modified ammonium polyphosphate. However, the modified ammonium polyphosphate disclosed therein has a high solubility in water; hence its use as a component of flame-retardants raises a problem.
Japanese patent publication No. Sho 52-39930 discloses a process wherein powder of ammonium polyphosphate (100 parts by weight) is uniformly mixed with powder of melamine (60 parts by weight), followed by heating the mixture at 320.degree. C., cooling and crushing the resulting melt to obtain powder of a melamine-added ammonium polyphosphate. However, as to the powder of melamine-added ammonium polyphosphate obtained according to the process disclosed in the above publication, since the melt is crushed, the surfaces of particles of ammonium polyphosphate are not uniformly coated by melamine, and the product is still insufficient in hygroscopicity, water-solubility and hydrolytic properties, and moreover it is necessary to crush the melt.
Further, Japanese patent applications laid-open No. Sho 59-207819 and No. Sho 61-103962 disclose that ammonium polyphosphate is microcapsulated using a thermo-setting resin such as melamine/formaldehyde resin to improve hygroscopicity, water-solubility and hydrolytic properties. However, according to such a process, since the use of a wet slurry constitutes a condition of its production and further since a thermosetting resin is used, when the ammonium polyphosphate is added to a thermoplastic resin or the like, there is raised a drawback that the resulting product is colored due to the heat deterioration of the thermosetting resin.
Problem to Be Solved by the Invention:
The present inventors have made extensive research in order to obtain a powdery ammonium polyphosphate which is not hygroscopic, difficultly soluble in water and hardly susceptible to hydrolysis, and as a result, have found that a powdery, melamine-coated ammonium polyphosphate obtained by adding and/or adhering melamine onto the surface of powdery ammonium polyphosphate particles by sublimation of melamine, constitutes a powdery ammonium polyphosphate provided with the above-mentioned properties, and at the same time, when the thus obtained powdery, melamine-coated ammonium polyphosphate is added as a component of a flame-retardant, to a thermoplastic resin or the like, ammonia gas does not evolve due to decomposition even if the product is exposed to the molding temperature of the thermoplastic resin, and yet melamine is added and/or adhered to the surface of ammonium polyphosphate particles by its sublimation; and according to the above production process, the powdery ammonium polyphosphate particles as a raw material barely agglomerate during the process of coating melamine to thereby obtain a melamine-coated ammonium polyphosphate keeping the initial particle size; hence it is unnecessary to further crush the ammonium polyphosphate. The present invention has been completed based upon the above-mentioned findings.
As apparent from the foregoing, the object of the present invention is to provide a melamine-coated ammonium polyphosphate which is not hygroscopic, barely soluble in water and hardly susceptible to hydrolysis, and a process for producing a melamine-coated ammonium polyphosphate which is in the form of powder even when not crushed.